The present invention relates to a rotary power transmission arrangement of the type which permits relative rotary motion between an input and an output element when a given level of torque is exceeded.
Torque limiting clutches, often called slip clutches, are commonly used as overload protection devices to prevent damage to members of a drive train when an output member is overloaded. They find wide application in agricultural machine drives, such as those of combine harvesters, where wide variations in rate of feeding and condition of crop material must be tolerated and overloading, including plugging resulting in the arresting of the motion of a crop engaging element, such as a conveyor, sometimes occurs. Without limiting the scope of the invention, it is convenient to discuss the field in terms of a particular application, such as power transmission between a shaft and a tangentially loaded element, such as a chain sprocket, spur gear or V-belt sheave concentric with it.
In a common form of conventional clutch, the so-called jump clutch, the connecting clutch elements are a pair of opposed serrated annular rings concentric with the shaft, one engaging the sprocket for rotation with it, the sprocket otherwise being free to rotate relative to the shaft, while the other ring is coupled to the shaft for rotation with it by means of a keyway or spline. The coupling of the sprocket to the shaft is maintained by an axial bias, typically by a compression spring holding the annular rings in engagement, and slipping of the clutch occurs only when the torque transmitted is sufficient to overcome the friction or torsional resistance derived from the mating serrations of the opposing clutch rings.
Slipping of the clutch clearly depends therefore on relative axial movement between components and the most common arrangement has been for the clutch half coupled to the shaft to remain axially fixed while the other half, drivingly connected to the sprocket, is displaced axially to permit relative rotational movement. The disadvantage of this arrangement is that with, for example chain drives of short center distance, the chain and sprocket are subject to misalignment and severe vibration while under heavy load tending to shorten the life of chain and sprocket severely. The alternative, accommodating the required axial displacement in the coupling of the other clutch half to the shaft, requires a relatively costly and sophisticated design permitting sliding (axial displacement) between surfaces under extreme torsional load.